1. Field of the Invention
The present invention relates to a synchromesh automatic transmission for selecting a shift lug of a target shift stage from a plurality of shift lugs aligned in a row and displacing the shift lug in a shift direction by a shift finger.
2. Description of the Related Art
An automatic transmission of a vehicle (car) has a structure to carry out transmission of a target shift stage by using a synchromesh.
In such a synchromesh automatic transmission, a structure is used in which a shift lug is provided to a plurality of shift forks for carrying out gear change of various shift stages, respectively, and by using a shift finger, only one shift lug corresponding to a target shift stage is selected from among shift lugs so as to displace the shift lug in a shift direction. In many cases, by providing a selector shaft, which can be displaced in the same direction as the direction of a row of shift lugs (namely, a direction to select the shift lug) and in a direction to shift the shift lug, a shift selector mechanism having a shift finger provided to this selector shaft is used. By selectively engaging the shift finger with one shift lug, namely, engaging the shift finger with the shift lug of the shift fork corresponding to the target shift stage by a motion in a selection direction of the selector shaft, and by displacing the selected shift lug in a shift direction by a motion in a shift direction of the selector shaft, shift of the target shift stage is achieved.
In such an automatic transmission, during selection of the shift lug of the target shift stage, in order to prevent transmission of a power through a shift stage that is different from the target shift stage, by using an interlock mechanism, the shift other than the target shift stage is prevented although the shift of the target shift stage can be carried out (in order to achieve the shift transmission of the target shift stage).
According to such an interlock mechanism, as shown in FIGS. 2 and 20 of Jpn. PCT National Publication No. 2004-518918, by assembling an interlock cam and an interlock plate in a selector shaft itself which displaces a shift finger in a shift direction and selecting a shift lug of a target shift stage, the interlock cam and the interlock plate are engaged with the shift lug of the shift stage other than the target shift stage, and the shift fork of the shift stage other than the target shift stage is controlled so as to be located at a neutral position. This kind of technique is disclosed in Jpn. PCT National Publication No. 2004-518918.
The structure to provide the interlock cam and the interlock plate on the selector shaft for displacing the shift lug to the shift position as described above is required to engage the interlock cam and the interlock plate with a shift lug of a shift stage other than the target shift stage in consideration of movement of the shift finger in a shift direction.
In order to achieve this engagement, a very high degree of accuracy is required. Therefore, in the above-described structure, the interlock cam and the interlock plate may be arranged at a position deviated from the shift lug to be engaged due to an error of each part or the like, and according to the circumstances, it is feared that the shift fork cannot be controlled so as to be located at the neutral position. Further, the circumstances to control the shift lug in the interlock cam and the interlock plate are changed due to movement of the shift finger in the shift direction. Therefore, this structure has a problem that, if the movement of the shift lug in the shift direction is changed, for example, for each kind of transmission, a structure to enable movement in the shift direction is required for each transmission and thus it is difficult to cope with each kind of transmission.